Effect of barrier thickness on electromigration reliability of Cu/porous low k interconnects

The effects of Ta barrier thickness on electromigration reliability of dual-damascene Cu/porous methylsilsesquioxane interconnects were investigated. With decreasing Ta barrier thicknesses, the threshold product of current density and line length (jL)c was found to be reduced due to less structural confinement from thinner barriers. The effect can be accounted for by the effective modulus of the structure except for the 75-A Ta barrier where the (jL)c product is reduced more than expected, probably due to the presence of defects in the barrier. Results from the early failure test structures revealed a bimodal failure distribution for samples with the 75-A and l00-A barriers. Focused ion beam microprobe and transmission electron microscopy observations revealed that the weak-mode early failure was caused by Cu outdiffusion through structural defects in the thin Ta barrier.